Solid ink or phase change ink printers conventionally receive ink in a solid form, either as pellets or as ink sticks. The solid ink pellets or ink sticks are placed in a feed chute and a feed mechanism delivers the solid ink to a heater assembly. Solid ink sticks are either gravity fed or pushed by a mechanism through the feed chute toward a heater plate in the heater assembly. The heater plate melts the solid ink impinging on the plate into a liquid that is delivered to a print head for jetting onto a recording medium.
In known printing systems having an intermediate imaging member, the print process includes an imaging phase, a transfer phase, and an overhead phase. In ink printing systems, the imaging phase is the portion of the print process in which the ink is expelled through the piezoelectric elements comprising the print head in an image pattern onto the print drum or other intermediate imaging member. The transfer or transfix phase is the portion of the print process in which the ink image on the print drum is transferred to the recording medium. The image transfer typically occurs by bringing a transfer roller into contact with the image member to form a nip. A recording medium arrives at the nip as the print drum rotates the image through the nip. The pressure in the nip helps transfer the malleable image inks from the print drum to the recording medium. In the overhead phase, the trailing edge of the recording medium passes out of the nip and the transfer roller is released from contacting the image member. Because the rotation of the transfer roller is driven by the rotation of the print drum, releasing the transfer roller from the image member substantially reduces the load on the electrical motor driving the image member. In this manner, the electrical energy consumed by the motor is reduced.
Printing may be performed in a simplex or duplex manner. Simplex printing occurs as an image is transferred from the image member to one side only of the recording medium. Duplex printing involves printing an image on each side of the recording medium. In duplex printing, the recording medium passes through the nip between the transfer roller and the print drum. The recording medium then is directed into a path that returns the recording medium to the nip so the side that was not printed during the first pass faces the print drum. As the recording medium goes through the nip the second time, an image is transferred to the unprinted side of the recording medium. The recording medium then exits the nip and is routed to the output tray. Additionally, treatment of the printed recording medium may occur as the printed medium progresses from the transfer nip to the output tray.
One issue that arises during duplex printing in ink printers is a condition called ghosting. To facilitate transfer of an ink image from a print drum to a recording medium, a drum maintenance system is provided to apply release agent to the surface of the print drum before ink is ejected onto the print drum. Release agent is typically silicone oil that is applied to the print drum by an applicator roll in the drum maintenance system that may be partially submerged in a release agent sump. A blade may be positioned at a location following the drum maintenance system to remove excess release agent from the print drum. The release agent provides a thin layer on which an image is formed so the image does not adhere to the print drum. During a series of simplex print operations, the transfer roller obtains little, if any, release agent from the print drum as the transfer roller is primarily in contact with the unprinted side of the recording medium. Consequently, the transfer roller effectively “dries out.” In this condition, the transfer roller may acquire ink from the printed side of the recording medium as it passes through the nip during the second pass of a duplex printing. That is, the relatively dry state of the transfer roller and the pressure in the nip may cause some of the ink and/or release agent from the side of the recording medium printed during the first pass of the medium through the nip in a duplex operation to migrate to the transfer roller. The presence of ink or release agent on an otherwise dry transfer roller produces a non-uniform surface on the transfer roller. On the next revolution of the transfer roller, the non-uniformity of the transfer roller causes different adhesion of the ink in the next image to the next recording medium that enters the transfer nip. The appearance caused by the different adhesions is sometimes called ghosting.
In an effort to address ghosting, attempts have been made to apply release agent to the transfer roller prior to commencing a duplex printing operation. One way of applying release agent to the transfer roller is to allow the transfer roller and image member to rotate together for one full revolution of the transfer roller before commencing the duplex operation. The amount of the release agent applied in this manner has been found to be generally insufficient. Allowing the transfer roller and the image member to rotate together for multiple revolutions has also provided unsatisfactory results because the transfer roller sometimes obtains too much release agent and transfer of the image to the recording medium during the duplex operation is adversely impacted. Another approach requires interruption of a series of simplex prints to apply release agent to the transfer roller using the print drum. Interrupting a series of simplex printings to apply release agent reduces the throughput of the printer and increases the time to process a queue of simplex printings. Being able to apply enough release agent to a transfer roller to reduce ghosting effects during duplex operations without reducing productivity during simplex operations is desirable in solid ink printers.